Lung-specific gene products include the lung surfactant proteins SP-A, SP-B, SP-C, SP-D, and Clara cell secretory protein (CCSP). The recent cloning of these gene products, the determination of their expression patterns in vivo (Weaver, et al., Biochem. J., Vol. 273, page 249-264 (1991); Wert, et al., Dev. Biol., Vol. 156, pgs. 426-443 (1993); Stripp, et al., Genomics, Vol. 20, pgs. 27-35 (1994)); and the characterization of cell lines that support their expression (O'Reilly, et al., Biochem. Biophys. Acta, Vol. 970, pgs. 194-204 (1988); Gazdar, et al., Cancer Res., Vol. 50, pgs. 5481-5487 (1990); Wikenheiser, et al., Proc. Nat. Acad. Sci. USA, Vol. 90, pgs. 11029-11033 (1993)) provide a model system to investigate the mechanisms involved in lung-specific gene expression.
The control of tissue-specific gene expression is thought to occur largely at the level of transcription initiation. Consistent with this observation is that appropriate cis-active sequences from tissue-specific genes often are sufficient to target expression of a reporter gene to the tissue of origin in vivo. (Jaenisch, Science, Vol. 240, pgs. 1468-1474 (1988).) Studies have shown that DNA-binding proteins interact specifically with these sequences to stimulate gene transcription (Maniatis, et al., Science, Vol. 236, pgs. 1237-1244 (1987): Mitchell, et al., Science, Vol. 245, pgs. 371-378 (1989); Johnson, et al., Ann. Rev. Biochem., Vol. 58, pgs. 799-839 (1989).) Liver-specific cis-active elements have been studied extensively, and several transcription factors including HNF-1, HNF-3, HNF-4, C/EBP, and DBP (Simmons, et al., Genes & Dev., Vol. 4, pgs. 695-711 (1990)) bind these regions and appear to act together to regulate transcription of liver-specific genes (Costa, et al., Mol. Cell. Biol., Vol. 9, pgs. 1415-1425 (1991)). None of these proteins appears to be restricted to liver cells. (Xanthopoulus, et al., Proc. Nat. Acad. Sci. USA, Vol. 88, pgs. 3807-3811 (1991)). This suggests that mechanisms other than the restricted expression of a transcription factor to a single cell type are responsible for the tissue-specific activity of these genetic elements. This could involve interaction between DNA bound factors at a unique cis-active environment (Milos, et al, Genes and Dev., Vol. 6, pgs. 991-1004 (1992); Nerlov, et al., Genes and Dev., Vol. 8, pgs. 350-362 (1994)) or between a DNA bound factor and a non-DNA bound cofactor (Mendel, et al., J. Biol. Chem., Vol. 266, pgs. 677-680 (1991)).
Recently, it has appeared that the mechanisms of transcriptional control of tissue-specific genes in the liver and lung may be related. This is suggested by the expression of HNF-3 and CCAAT enhancer binding protein-.alpha. (C/EBP) family members in the lung, (Lai, et al., Genes and Dev., Vol. 5, pgs. 416-427 (1991); Cao, et al., Genes & Dev., Vol. 5, pgs. 1538-1552 (1991); Xanthopoulus, et al., 1991), and by the finding that HNF-3 proteins bind to a region of the CCSP gene promoter in vitro (Sawaya, et al., Mol. Cell. Biol., Vol. 13, pgs. 3860-3871 (1993); Bingle, et al; Biochem J., Vol. 295, pgs. 227-232 (1993)).
Despite the work accomplished in the above studies, a need still exists to isolate and obtain genetic elements which will direct lung cell specific expression of genes of interest.